Dental impression materials, e.g., VPS impression materials, are common products which dentists use to make highly precise impressions of patients' teeth. One disadvantage of some impression materials is their hydrophobic nature which may negatively impact the achievable accuracy of the detail of an impression under the moist conditions in a patients mouth. To overcome this problem, surfactants have been added to VPS impression materials to make those more hydrophilic. Many so called “hydrophilic” VPS impression materials are available in the market based on this technology.
Hydrophilicity of an impression material can be determined by a measurement of contact angles of drops of water on the surface of a sample of the impression material either in the unset or in the set phase using a standard goniometer. Generally, the higher the hydrophilicity of a VPS impression material, the lower the contact angle is. As the addition of surfactant can affect hydrophilicity, the amount of surfactant can influence the degree of hydrophilicity and the contact angle of dental materials such as VPS impression materials.
Generally, it is believed that the hydrophilicity of an impression material should be as high as possible, so there has been a tendency to develop VPS dental impression materials with increased amounts of surfactant to achieve as low contact angles as possible. However, simply increasing the amount of surfactant in a VPS impression material can cause other problems, for example, with respect to aging properties of the resulting dental materials.
Typically, VPS impression materials consist of two components—a base paste and a catalyst paste—the latter including a highly reactive platinum catalyst. These pastes are often filled into and stored in cartridges or foil bags. When a surfactant is present in the catalyst paste, interactions between the surfactant and the platinum catalyst may be observed, which are believed to retard the curing reaction and decrease the shelf life of the catalyst paste. Additionally, the properties of the cured material may be negatively influenced by such an interaction. Accordingly, it is usually necessary, to include only low amounts of surfactant, if any at all, in the catalyst paste, and to include most or all of the surfactant in the base paste.
The base paste of a VPS impression materials can also contain vinyl polysiloxanes, polysiloxanes or oligosiloxanes with Si—H-groups and additives like pigments, surfactants, plasticizers, retarders, etc. Without the presence of a platinum catalyst, the base paste should not cure because the platinum catalyst induces the hydrosilylation curing reaction of the curable components.
The retardation of curing in crosslinkable mixtures is described in U.S. Pat. No. 6,346,562 B1 and U.S. Pat. No. 6,300,455 B1. Both patents describe certain addition-crosslinkable silicone rubber systems. A generally encountered problem with such systems is, according to U.S. Pat. No. 6,346,562 B1 and U.S. Pat. No. 6,300,455 B1, that once the reactive mixture has been prepared it will cure, even at room temperature. This is particularly problematic when the production machines to make the base and catalyst pastes are not running for a relatively long time as a result of technical malfunctions or other causes. In such cases, the reactive silicone rubber mixture present in the machines can crosslink at room temperature, clogging the machines and necessitating very costly cleaning work before the machines can be restarted. For this reason, U.S. Pat. No. 6,346,562 B1 suggests the addition of at least one phosphorus compound to the reaction mixture in order to suppress curing of the mixture at room temperature. The document neither relates to hydrophilized dental materials, nor to materials designed to crosslink at room temperature.